This invention relates generally to rotary machines and more particularly, to methods and apparatus for attaching wind turbine blades to a hub.
Generally, a wind turbine includes a rotor having multiple blades. The rotor is mounted on a housing, or nacelle, that is positioned on top of a truss or tubular tower. Utility grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) can have large rotors, e.g., 30 meters (m) (98 feet (ft)) or more in diameter. Blades, attached to rotatable hubs on these rotors, transform mechanical wind energy into a mechanical rotational torque that drives one or more generators. The generators are generally, but not always, rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert the rotational mechanical energy to electrical energy, which is fed into a utility grid. Gearless direct drive turbines also exist.
Contemporary blades are typically at least partially fabricated of a laminated (i.e., layered) fiber/resin composite material. In general, reinforcing fibers are deposited into a resin within a range of predetermined orientations. The fiber orientations are often determined by a range of expected stress factors that a blade may experience during an expected blade lifetime. The planar regions between the laminations are often referred to as interlaminar regions and are normally the weakest element of a composite material. Loads are normally carried in the planes of the laminations and such loads are transferred from the planes of the lamina to an attachment or interface with another component, i.e., the hub. This transfer typically occurs via interlaminar shear, tension, compression, or a combination thereof. As a consequence, when load within the laminar planes is increased, stress on the interlaminar regions increases as well. In the event that an interlaminar shear stress limit (i.e., the shearing force tending to produce displacement between two lamina along the plane of their interface) of an interlamination region is exceeded, the potential for delamination (the separation of a laminated material along the plane of the interlaminar regions) is increased. Delamination may lead to material strain, i.e., elastic deformation of a material as a result of stress.
Some examples of stress factors are vertical wind shear, localized turbulence (including interaction of the rotor with the tower), gravity, wind flow variations and start-stop cycles. Vertical wind shear is typically defined as the relationship between wind speeds and height above the surface of the earth, i.e., altitude. In general, as the altitude increases, wind speed increases. Given a blade of 30 meters (98 ft) or more, and the subsequent large diameter of rotation (at least twice the blade length plus the diameter of the associated hub), wind speed can increase 5% to 10% above that at the hub centerline from the hub centerline up to the end of the blade at the blade tip with the blade pointing straight upward. The wind speed may also decrease 5% to 10% below that at the hub centerline from the hub centerline down to the end of the blade at the blade tip with the blade pointing straight downward. As the blades rotate, the cyclic increasing and decreasing of the wind shear induces a cyclic in-plane or interlaminar stress within the blades.
Localized turbulence includes stationary wakes and bow waves induced by the blades and by the near proximity of the rotating blades to the tower. As the blades rotate through these localized regions, additional stresses are induced within the blades. Also, as the blades rotate, gravity induces fluctuating bending moments within the blades that also induce in-plane and interlaminar stresses. Cyclic acceleration and deceleration of the blades due to the aforementioned wind flow variations and start-stop cycles induce cyclic stresses on the blades as well.
The blades are typically designed and manufactured to withstand such stresses including the cumulative impact of such stresses in a variety of combinations. The blades are also designed and manufactured to withstand the cumulative impact of a predetermined number of stress cycles, commonly referred to as fatigue cycles. Upon exceeding the predetermined number of fatigue cycles, the potential for material delamination may increase.
As described above, blades are typically attached to a rotating hub at attachment regions designed and fabricated to receive the blades. The blades also have integral attachment regions. The hub and the blade attachment regions act as load transfer regions. For example, the weight of the blades and the aforementioned cyclic stresses are transferred to the hub attachment regions via the blade attachment regions. Also, as described above, the majority of the load is carried through the planes of the laminations except in the immediate vicinity of attachments and interfaces. As blade size and weight increase, the laminations of the blade attachment regions may have an increased potential for exceeding interlaminar shear stress limits.